1. Field of Invention
The present invention relates generally to lithographic projection apparatus.
2. Related Art
The term “patterning device” as here employed should be broadly interpreted as referring to devices that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning devices include:                A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.        A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic elements. More information on such mirror arrays can be gleaned, for example, from U.S. patents U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.        A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.        
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning devices as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
One of the most challenging requirements for micro-lithography for the production of integrated circuits as well as liquid crystal display panels is the positioning of tables. For example, sub-100 nm lithography demands substrate- and mask-positioning stages with dynamic accuracy and matching between machines to the order of 1 nm in all 6 degrees of freedom (DOF), at velocities of up to 2 ms−1.
A popular approach to such demanding positioning requirements is to sub-divide the stage positioning architecture into a coarse positioning module (e.g. an X-Y table or a gantry table) with micrometer accuracies but traveling over the entire working range, onto which is cascaded a fine positioning module. The latter is responsible for correcting for the residual error of the coarse positioning module to the last few nanometers, but only needs to accommodate a very limited range of travel. Commonly used actuators for such nano-positioning include piezoelectric actuators or voice-coil type electromagnetic actuators. While positioning in the fine module is usually effected in all 6 DOF, large-range motions are rarely required for more than 2 DOF, thus easing the design of the coarse module considerably.
The micrometer accuracy required for the coarse positioning can be readily achieved using relatively simple position sensors, such as optical or magnetic incremental encoders. These can be single-axis devices with measurement in one DOF, or more recently multiple (up to 3) DOF devices such as those described by Schaffel et al. “Integrated electro-dynamic multicoordinate drives”, Proc. ASPE Annual Meeting, California, USA, 1996, p. 456-461. Similar encoders are also available commercially, e.g., position measurement system Type PP281R manufactured by Dr. J. Heidenhain GmbH. Although such sensors can provide sub-micrometer level resolution without difficulty, absolute accuracy and in particular thermal stability over long travel ranges are not readily achievable.
Position measurement for the mask and substrate tables at the end of the fine positioning module, on the other hand, has to be performed in all 6 DOF to sub-nanometer resolution, with nanometer accuracy and stability over the entire working range. This is commonly achieved using multi-axis interferometers to measure displacements in all 6 DOF, with redundant axes for additional calibration functions (e.g. calibrations of interferometer mirror flatness on the substrate table).
Although the technology behind such interferometer systems is very mature, their application is not without problems. One of the most significant drawbacks of the interferometer is the dependence of wavelength on environmental pressure and temperature, as described by Schellekens P. H. J. “Absolute measurement accuracy of technical laser interferometers” Ph.D. Thesis, TU Eindhoven, 1986, which is given by:
                              λ          a                =                              λ            v                    η                                    (        1        )            where:
                                          (                          η              -              1                        )                                P            ,            T            ,            H            ,            C                          =                                            D              ×              0.104126              ×                                                10                                      -                    4                                                  ·                P                                                    1              +                              0.3671                ×                                                      10                                          -                      2                                                        ·                  T                                                              -                      0.42066            ×                                          10                                  -                  9                                            ·              H                                                          (        2        )            D=0.27651754×10−3×[1+53.5×10−8(C−300)]    P: atmospheric pressure [Pa]    T: atmospheric temperature [° C.]    H: water vapor pressure [Pa]    C: CO2 content [ppm]
This remains one of the major problems in the thermal design of an optical lithography system. Typically, both temperature and pressure along the optical path of the interferometer has to be actively controlled to mK and mbar levels by the use of dry, clean (to better than Class 1) air, e.g. supplied by air showers.
In addition, the mounting adjustment of multi-axis interferometers for orthogonality and coplanarity, as well as the subsequent calibration procedure to remove any residual errors, are both extremely complex and time consuming. Even after such adjustments and calibration procedures, the measurement is only accurate if the relative positions of the interferometer blocks remain stable. The nanometer dimensional stability requirements of the metrology frame, on which the interferometer blocks are mounted, imply that the metrology frame has either to be made out of a material with low or zero coefficient of thermal expansion (CTE), such as Invar or Zerodur, or active thermal stabilization to mK levels, or both. Furthermore, the pointing stability of the laser beam during operation may introduce additional cosine or Abbe errors which need to be calibrated out on a regular basis by some form of automated routine.
An interferometer system is of course only a relative measuring system, capable of measuring changes in length (of optical path, to be precise). A zero reference in each degree of freedom can only be generated with additional equipment, such as so-called alignment sensors as described in WO 98/39689.
Although metrology frames in state-of-the-art lithography systems are highly isolated from ambient vibration, thermal deformation of the order of 0.5.times.10.sup.-9 m is not totally avoidable. It is, therefore, desirable that the position of the substrate or mask tables be measured directly relative to the optical imaging system. Mounting of interferometers directly on the lens, for example, is both difficult and undesirable. Relative length measurement to the lens can, however, still be realized by differential interferometry, at the expense of the added complication and cost.
The multiple beams required for such 6 DOF interferometric measurement cannot be adequately supplied with sufficient optical power by one laser source, thus requiring multiple sources with additional wavelength matching demands. The total thermal dissipation of the lasers and detectors combined exceeds 50 W, which is well above the level allowable for the dimensional stability of the metrology frame. Both the lasers and the detectors have thus to be mounted remotely via optical links.
As can be seen, whilst the resulting interferometry based system is technically viable and has been implemented in practice, it is by no means simple, robust and economical.
The most obvious alternative to interferometers for long-range displacement measurements with micrometer or nanometer resolutions is the optical incremental encoder. Optical encoders with sub-nanometer resolutions have become available in recent years and have been promoted as viable alternatives to single-axis interferometry. The sub-nanometer resolution is achieved by using fine-pitched gratings (down to 512 nm) in combination with interpolation techniques (up to 4096×). Most of such encoders, however, provide length measurement in 1 DOF only. As such, they do not lend themselves readily to nano-metrology in all 6 DOF simultaneously. Amongst the difficulties is the high level of crosstalk of the displacement signal to parasitic movements in the other 5 DOF.